The present invention is directed to an optical transceiver module.
Optical transmitters are a well-known and essential optical component used in data storage and telecommunications networking applications. Typically, the components required to convert electrical to optical signals are referred to as Transmitter Optical Sub-Assemblies (TOSAs). The optical output of a TOSA is typically focused into an optical fiber. Once the optical signals reach their destination, they are typically focused into Receiver Optical Sub-Assemblies (ROSAs) for conversion back into corresponding electrical signals.
A TOSA typically includes a laser diode for producing an optical signal and a lens for focusing the optical signal into the input end of an optical fiber. Laser diodes (e.g. distributed feed-back laser diodes) are typically sensitive to back-reflected light (e.g. light reflected off of the input face of the optical fiber back into the laser diode). Therefore, TOSAs also typically include an optical isolator that passes the output optical signal from the laser diode to the optical fiber, but prevents any back-reflected light from reaching the laser diode.
Transceiver modules for fiber-based communications applications typically contain one or more TOSAs and ROSAs along with associated drive electronics, and each of these components must conform to a variety of performance criteria specified over a wide operating temperature range. These performance criteria include average power output, extinction ratio, eye diagram mask margin, and speed/bit rate. Various schemes may be employed at the module level to adjust the ac swing and the dc current bias to the laser diode to compensate for temperature sensitivity of certain laser diode parameters, hence maintaining the module specifications over the operating temperature range. To improve laser speed characteristics, the diode laser is usually biased at an average dc bias current above the lasing threshold current. FIG. 1 shows a sketch of an exemplary P v. I curve indicating the threshold current (Ith) and a dc bias current (Iop). An ac modulation is applied to generate the low power 0 bits (shown as P=0 in FIG. 1) and high power 1 bits (shown as P=1 in FIG. 1). Other schemes such as a return to zero bit encoding scheme for which bits are sent as pulses of light may be used instead for other applications, for example, when dispersion is critical. The dc current is often controlled using a constant power feedback loop, stabilizing the current detected in the monitor photodiode within the laser diode package.
The ability of the laser diode to perform at the required speed depends on the dc current bias (dc drive current), where generally the speed performance increases as the de drive current is increased. Traditional methods used for selecting the operating dc drive current of the laser diode include: a) adjusting the dc drive current until the output power reaches a desired value, and b) adjusting the dc drive current to a fixed offset above the laser diode's threshold current Ith. Unfortunately, neither of these procedures guarantees that the dc drive current is sufficient to achieve the required speed performance at each temperature throughout the operating range.
In addition to the minimum speed requirement, a laser has to satisfy a boundary imposed by the laser diode's natural relaxation oscillation resonance frequency. A laser diode has a relaxation oscillation resonance frequency f0 at which it is naturally inclined to resonate, limiting its useful bandwidth. If this resonance frequency occurs within the bandwidth required by the data rate, the resonance will be excited upon data transmission. The resonance frequency, if excited, may cause overshoot and ringing on the signal, possibly eye diagram mask hits indicative of bit errors and hence module failures. Therefore, the relaxation resonance frequency f0 serves as an upper boundary on the maximum usable bandwidth of the laser, limiting the bit rate at which the laser can operate. The square of the relaxation resonance frequency f0 is proportional to the dc drive current less the threshold current, and inversely proportional to the active volume of the device. See, for example, Small-Signal Frequency Response of Long-Wavelength Vertical-Cavity Lasers, IEEE Photonics Technology Letters, Vol. 13, No. 10, October 2001. By merely selecting the dc drive current based solely on a desired power level or on a fixed offset above the threshold current, there is no guarantee that the relaxation resonance frequency will not fall within the operating bandwidth of the laser diode. Thus, unless the resonance frequency is taken into account, the laser diode may fail to meet its speed performance requirement over the entire range of operating temperatures. Although the laser speed increases with dc bias current, the bias current can not be increased without bound because a) laser diode lifetime decreases with increased bias current, b) laser drive circuitry has limited available output current, c) increasing the dc bias also increases the ac swing needed to maintain the specified extinction ratio ER [dB] that is defined in terms of the ratio of the module optical power levels for the one and zero bits (ER=10 log (P1/P0)), d) electromagnetic interference (EMI) increases proportionally to the ac swing, and e) the specified output power may be exceeded, requiring attenuation.
There is a need for a method of optimizing the operational dc drive current of a laser diode that not only produces the desired power but also the desired speed performance throughout the expected operating temperature range, without the time and expense of checking speed performance over temperature for each optimized laser diode.